Figures produced by the Irish Nurses and Midwives Organisation show that 7,392 patients admitted to hospital in June had to wait for a hospital bed.
The INMO says the figure is a 7% increase on the same month last year.
It said that on 25 June, 482 people were waiting on trolleys, a 49% increase on last year, and the highest number for any day in June since records began.
Among the hospitals the INMO says recorded their worst ever June for overcrowding were Cork University Hospital, South Tipperary General Hospital, University Hospital Waterford and Tallaght University Hospital.
INMO General Secretary Phil Ní Sheaghdha said: "Our members tell us that there's no longer a summer respite when it comes to overcrowding.
"Summer 2019 is as bad as winter was five years ago. Understaffing is driving year-round unsafe conditions.
"The Government and HSE need to get a handle on this problem. That means implementing the Safe Staffing Framework and making real progress on Sláintecare.
"Without serious investment and reform, this problem will continue to spiral, with patients and frontline staff paying the price."
Meanwhile, there has been a call for the Government to fill vacant consultant posts to decrease the number of people on outpatient waiting lists.
The Irish Hospital Consultants Association has said there are 550 consultant posts that are either unfilled or filled using "expensive" agency staff or locums.
Speaking on RTÉ's Today with Sean O'Rourke, IHCA vice president Laura Durcan said when you look at trolley figures, patients who wait to access care inevitably deteriorate and come to a crisis point.
She said there are bottlenecks in the healthcare system both when people try to see a consultant for the first time and also when accessing theatre times for procedures.
Speaking on the programme, Fine Gael Senator Colm Burke said that more hospitals need to be built to increase the supply of beds.
He also said that the number of people employed by the HSE has increased significantly since 2014.